1. Field of the Invention
The invention pertains to oscillator circuits, and more specifically to a multi-stage differential ring oscillator with automatic gain controller and low frequency shut off.
2. Description of Related Art
Oscillators are used in a variety of integrated circuits to generate clocking signals. One type of well known oscillator design is a multi-stage differential ring oscillator. Such an oscillator is generally constructed of a plurality of delay cells, delay cell gain adjust circuitry, and an output stage.
There are a number of concerns for power sensitive designs for ring oscillators. These concerns include dynamic power consumption, static power consumption, and a range of operability. Affecting dynamic power consumption and range of operability is the gain control circuitry. Generally, delay cells have outputs whose amplitudes vary with operating frequency. Gain adjust circuitry is provided to the delay cells to counteract that variance. Over the operating range of the oscillator, the gain adjust circuitry ensures the output levels will not vary by an excessive amount. This permits both the wide range of operation and prevents high dynamic power consumption in a situation where the delay cells might drive their outputs to excessive levels.
Prior ring oscillator designs often employed fairly complex automatic gain control circuitry that tracked the output levels of either an actual delay cell or a "dummy" delay cell to provide gain control to keep the output level constant. This feedback approach, however, typically required an extra differential amplifier and other additional circuitry.
Static power consumption can be affected when the oscillator is turned off. Some designs permit excessive current drain due to low frequency oscillations, even when the oscillator is off.